Inherit dominant-baseline into nested <tspan> elements

[StefanoD]

dominant-baseline was listed in is_non_inheritable, so usvg only let a <tspan> pick up the value from its direct parent element. For a nested <tspan> whose immediate parent does not carry the property, the value of an ancestor <text> was therefore lost and the span fell back to auto (alphabetic baseline). The nested run then sat on a different baseline than its siblings — the "weird" positioning reported in #864.

dominant-baseline is actually an inherited property, unlike its siblings alignment-baseline and baseline-shift:

• SVG 2 / CSS Inline Layout Module Level 3 define it with "Inherited: yes". https://www.w3.org/TR/SVG2/text.html#BaselineAlignmentProperties
• Chromium marks it inherited: true (field group svginherited), while alignment-baseline and baseline-shift are not inherited: https://github.com/chromium/chromium/blob/main/third_party/blink/renderer/core/css/css_properties.json5 (name: "dominant-baseline" ... inherited: true)
• Inkscape declares it as an inheriting SPIEnum (default inherits = true).

Removing it from is_non_inheritable makes find_attribute/resolve_inherit walk the full ancestor chain, matching Chrome's rendering (verified by comparison) and the spec.

Fixes #864.

Generated with Claude

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Note: I accidentally closed the original PR (#1070) by deleting my fork, which auto-closed it. This reopens the same change — the branch and commits are unchanged.

[luisbg]

lgtm

Approach is correct but I'm not sure the comment block in crates/usvg/src/parser/svgtree/mod.rs is needed.

[StefanoD]

I won't have the time to continue with the PR. You can take it over.

[LaurenzV]

Thanks!